FIG. 1 is a schematic configuration of a conventional direct-conversion receiver using a sub-harmonic frequency mixer. The direct-conversion receiver shown in FIG. 1 comprises a poly-phase filter 130, an LNA 150, an in-phase channel frequency mixer 110 and a quadrature phase channel frequency mixer 120. The poly-phase filter 130 generates LO signals for in-phase channels of 0°, 90°, 180° and 270°, and LO signals for quadrature channels of 45°, 135°, 225° and 315°. The LO signals for the in-phase channels generated by the poly-phase filter 130 are provided to the in-phase channel frequency mixer 110 to produce +I and −I signals as in-phase baseband signals. The LNA 150 amplifies an RF signal received thereto while minimizing noise.
Meanwhile, the LO signals for the quadrature phase channels generated by the poly-phase filter 130 are provided to the quadrature phase channel frequency mixer 120 to create +Q and −Q signals as quadrature phase baseband signals.
The poly-phase filter 130 is composed of a multi-stage resistor-capacitor circuit that causes large loss, which results in a large power loss in the LO signals. Hence, in order to compensate the power loss in the LO signals caused by the poly-phase filter 130, an LO amplifier 140 is arranged before or after of the poly-phase filter 130 for each phase of the LO signals. Such an additional LO amplifier 140, 171, 172, 174 and 177 makes not only the whole receiver structure complicated but also the power consumption high.
FIG. 2 presents a detailed circuitry diagram of any one of the sub-harmonic frequency mixers, e.g., 110 depicted in FIG. 1.
Substantially, such a type of the sub-harmonic frequency mixer 100 has been proposed to remedy a dc-offset problem with an existing direct down-conversion transceiver wherein the RF signal and the LO signal have the same frequency. In the sub-harmonic frequency mixer 100, a frequency of the LO signal is set to have half that of the RF signal. This sub-harmonic frequency mixer 110 performs a frequency conversion of the RF signal using a second harmonic component of the LO signal.
The sub-harmonic frequency mixer 110 has a stacked configuration of an RF signal input unit 112 and an LO signal input unit 114 that are composed of transistors. In the sub-harmonic frequency mixer 110, in order to produce the in-phase baseband signals +I and −I, there are required four LO signals of 0°, 90°, 180° and 270° having 90° phase difference with each other, and of two RF signals having 180° phase difference with each other. Further, in order to produce the quadrature phase baseband signals +Q and −Q, there are needed four LO signals of 45°, 135°, 225° and 315° having phase difference of 90° with each other, in addition to the four LO signals having 90° phase difference as set forth above. Thus, a separate mixer for quadrature phase channel frequency is needed thereto. As a result, total eight LO signals having 45° phase difference with each other and two RF signals having 180° phase difference with each other are necessary to obtain the entire baseband signals +I, −I, +Q and −Q.
As described above, the sub-harmonic frequency mixers 110 and 120 employed in the direct-conversion receiver 100 has a plurality of transistors stacked in a multi-stage fashion. However, such a multi-stage fashion can not guarantee a sufficient voltage swing at its output end under a limited supply voltage and thus makes the inevitable use of high supply voltage.